1. Field of the Invention
This invention relates to a method of making feedstock for the formation of articles from fine particles and to the article itself.
2. Brief Description of the Prior Art
In the prior art procedures for formation of articles from powdered metals, two distinct approaches have been taken, the older approach being that of press and sinter and the more recent approach being that of molding, debinding and sintering of a thermoplastic feedstock containing small particles or aggregate and a binder. Examples of the latter procedures are set forth in U.S. Pat. Nos. 2,939,199 (Strivens), 4,197,118 (Wiech), 4,404,166 (Wiech), and 4,445,936 (Wiech) and Canadian Pat. No. 1,177,290 (Wiech). In order to achieve high sinter densities and high sinterability using the more recent technology described above, it has been necessary to utilize fine particles on the order of about 4 microns which have been blended with a thermoplastic binder as the feedstock material. After formation of the green article, the binder was removed and, by well known sintering relationships, the material could achieve a high final fired density with its attendant excellent properties of tensile strength and high elongations. The drawback with the use of fine particles, however, is that they tend to be very expensive as opposed to the much larger diameter particles employed in conventional powder metallurgy of the press and sinter type. It has therefore been an objective in the art to provide feedstock materials that have substantially the same molding and sintering properties as the fine particle feed stock meterials, yet which are more comparable in cost to the less expensive large particulate materials used in the press and sinter technology. These large particle size materials cannot presently be used successfully in the above described more recent technology.
The properties of thermoplastic molding feedstock materials are such that, during the green formation phase, the green feedstock material must behave as if it were a well behaved thermoplastic material. It must then be readily debound and must, under conventional sintering practice, be sinterable to high density with a non-interconnecting porosity. The final material must have a high elongation and high mechanical properties, generally speaking, better than ninety percent of the properties of an equivalent forged material. In the prior art, this has been attained by utilizing particles with diameters on the order of twenty-five percent of the diffusion length of the various chemical species that are involved in the sintering phenomenon. Efforts have been made to employ larger diameter particles by using the conventional concepts found in ceramic technology and powdered metallurgy of distributing particle sizes to maximize the green feedstock density of classifying the particles so that the smaller particles fit into the interstices of the larger particles. While this approach has demonstrated that a high engineering property material in ceramics and powdered metallurgy can be attained, it has been most successful in those systems in which the entire sintering forces are due to the free surface energy forces on the particles.
The prior art densification forces in compact and sinter powder metallurgy are those mechanical forces that collapse the particulate field together by mechanically yielding the particles and in which sintering serves only to weld the particles together. This result is because the particle sizes present in classical powder metallurgical applications cause the particle to particle diffusion field to be far less than the particle diameter. This causes the particles to weld together but does not achieve any substantial densification, i.e., the centers of the particles moving closer to each other by an exchange of material between the particles.
When particulate fields with particles that are about 25% of the diffusion length of the active diffusing chemical species are used, especially in metals though not limited to metals, solid state diffusion utilizing only the free surface energy of the particles as a driving force behind diffusion results in very high engineering property materials. This technique is currently being carried but on a commercial basis and is well known in the art as demonstrated in the above noted patents. A drawback to this technique, however, is that the cost of a given weight quantity of powder that has a very fine particle size with a very narrow particle size distribution is very costly when compared to the much larger particle sizes which are utilized in powdered metal technology of the press and sinter or compact and sinter type. The particle sizes that are presently employed for fine metal powders are approximately 4 microns in diameter with a distribution such that there are few particles larger than about 5 microns and few smaller than about 2 microns. Ideally, it is desired to have all particles of exactly the same size, however, as one deviates therefrom, the final densities of the final article produced after debinding and sintering become lower and the mechanical properties developed become lower also. In addition, the elongation decreases and the tensile strength decreases. A preferred range for the final particle aggregate is 4 microns plus or minus about 50% or less.